Growth of the graded zone and its impact on cutting performance in high-pressure nitrogen modified functionally gradient hardmetals

摘要

Nitrogen pressures of 5 and 25 bar were applied for a diffusional treatment at T = 1200 deg C to modify the surface and near-surface areas of two dense-sintered (Ti,W)(C,N)/Co-based alloy grades in order to develop functionally gradient hardmetals (FGHMs). Upon this nitrogen enrichment, titanium carbonitride-rich graded diffusion zones form in which the [N]/[C] ratio decreases towards the interior. From systematic experiments an expected parabolic layer growth rate for these zones was identified. The thickness of the zones is dependent on the nitrogen pressure and the nitrogen content of the starting formulation. Cutting tests were carried out under conditions suited to compare the FGHMs to conventional hardmetals and to determine the optimum layer thickness. A substantially better performance was obtained for the described FGHM type indicating increasing performance upon decreasing zone thickness to a value of only few micrometer.